The present invention relates to a pellicle for a photolithographic mask used for patterning of semiconductor devices. More particularly, the invention relates to a pellicle for covering a photolithographic mask used for patterning on a semiconductor wafer in the manufacturing process of LSIs, VLSIs and the like as well as liquid-crystal display units with an object to prevent dust deposition on the photomask.
It is a well established technology in the manufacturing process of LSIs, VLSIs, liquid-crystal display units and the like that a fine pattern is formed on the surface of a semiconductor silicon wafer or other substrate materials by the techniques of photolithography in which the surface of the substrate is exposed pattern-wise to light through a photomask bearing the pattern. A serious problem in this process is that the surface of the photomask is sometimes very susceptible to the deposition of dust which is responsible for the degradation of patterning quality such as deformation of the pattern, roughened edge line of the pattern, decrease in the image contrast and so on due to scattering of light by the dust particles.
It is therefore essential that the photolithographic patterning works are conducted in a clean room under circulation of air freed from dust particles as completely as possible although the problem of dust deposition cannot be completely solved even by this means. Accordingly, it is usually practiced that a pellicle, which is a thin film of a polymeric resin highly transparent to light such as nitrocellulose, cellulose acetate and the like, is applied to the surface of the photomask to prevent dust deposition thereon.
A pellicle formed from the above mentioned cellulose derivatives, however, has several problems and disadvantages. For example, the polymer is not highly transparent in the ultraviolet region of light used for the pattern-wise exposure having a relatively strong absorption in the short wavelength region of 210 to 400 nm. Moreover, the cellulose derivatives are rather unstable against irradiation with ultraviolet light so that yellowing occurs and cracks are eventually formed in the pellicle after a certain period of use. Therefore, pellicles made from a cellulose derivative are not quite satisfactory for the patterning works using short-wavelength ultraviolet light of so-called i-line or the light emitted from an excimer laser as the light source in fine patterning for the manufacture of VLSIs and very finely patterned liquid-crystal display units.
Besides the above mentioned cellulose derivatives as the material of the pellicle for a photolithographic mask, a proposal is made in Japanese Patent Kokai 60-83032 that a pellicle is made from a transparent film of a specific fluorocarbon resin which is a binary copolymer of tetrafluoroethylene and vinylidene fluoride or a ternary copolymer of tetrafluoroethylene, hexafluoropropylene and vinylidene fluoride. Although the pellicle of such a fluorocarbon resin has increased transparency to short-wavelength ultraviolet light at least as prepared, the polymer, which is amorphous and has a linear molecular structure, has a tendency toward subsequent crystallization under irradiation with ultraviolet light forming crystallites which more or less cause scattering of light to gradually decrease the transparency of the pellicle during use.
As a solution for the above mentioned problem of subsequent crystallization in the pellicle made from a fluorocarbon resin, Japanese Patent Kokai 3-39963 proposes a pellicle made from a fluorocarbon resin which is a copolymer of tetrafluoroethylene and a fluorocarbon monomer having a cyclic perfluoro ether group. This fluorocarbon resin is indeed effective in solving the problem of the subsequent crystallization but has another difficult problem that the pellicle of the resin is highly susceptible to the accumulation of electrostatic charges on the surface. Needless to say, dust particles are readily attracted by and deposited on the electrostatically charged surface so that the purpose of the pellicle for preventing dust deposition can no longer be met once the pellicle film is electrostatically charged to attract and cause deposition of dust particles on the pellicle surface.